Original artworks (i.e., photographs, hand drawn pictures, hand or machine drawn advertising, computer generated images, etc.) are typically converted from continuous tone to half-tone (dot matrix format) in order to facilitate mass production of the work on a printing press. Color original artworks are generally separated into four half-tone color separations which are hereinafter denoted as "separations". Each half-tone separation carries image information of a separate one of the four colors of the artwork. In some instances an original color artwork is converted into five or more separations. The separation process can be achieved in a number of well known ways. A camera may be used to take a picture of images resulting when different color filters are placed in front of an art object which is illuminated. A graphic arts scanner may also be used to create the separations. The separations are typically formed on silver films. A color proof image (hereinafter a "proof") is produced from the separation films and is visually compared to the original artwork. The separations are approved if the proof is judged to be an acceptable reproduction of the original artwork. The image of each separation is then reproduced using well known techniques on a separate printing plate which typically is an aluminum sheet with an organic film coating. The printing plates are then used successively to print the contained images onto a medium such as paper, metal, plastic, or fabric. This medium containing an image reproduction of the original artwork is known as a press sheet. One example of a publication which generally uses high quality press sheets is an annual business report.
One problem is that even if the proof is a close reproduction of the original artwork, the final press copies may not be. There are on many occasions ascertainable variations in tone scale and color reproduction between the press copy and the original artwork which are not acceptable to the reviewer. One basic reason for the variations is that different printing presses and various media on which the final image is printed produce different amounts of "dot gain". Dot gain is the increase in the visually perceived size of printed dots over the actual size of corresponding dots on the separations. Accordingly, even though separations provide an acceptable proof, they do not in many cases provide an acceptable press sheet.
Various proof generating equipment is offered commercially. 3M Company markets, under its trademark "Color-Key", pre-press proofing materials involving pre-sensitized ink pigment coatings, in either transparent or opaque colors, on transparent polyester base sheets. Each pigment coating is intended to be correlated with various process color printing inks. Each primary color and black "Color-Key" sheet is overlaid by its associated separation negative and after exposure and development, the four "Color-Key" Sheets are overlaid in register to provide a "proof" or simulation of what the four-color work will look like when printed. The high contrast characteristic of a photopolymer used makes it extremely difficult to control dot size by varying illumination. It is also essentially impossible to control overall density because a given amount of the pigment is already dispersed in the photopolymer and therefore cannot be variably removed to decrease density. This pre-press proofing system falls well short of providing the user with a fully accurate proof in the sense of the colors, color registry and texture of the color work when a press prints with ink on paper.
3M Company also markets a pre-press color proofing system under the trademark "Transfer-Key", which is said to provide a complete four-color proof on a single sheet. In this system, factory pre-coated carrier sheets of color pigment, respectively bearing cyan, yellow, magenta and black pigment, are successively manipulated to laminate each pigment onto the base material by use of their proprietary laminator. Specifically, a cyan pigment layer of the carrier sheet is first laminated to the base material, which is then exposed to the cyan separation negative and the sheet then developed in a proprietary processor. The same lamination/exposure/development cycle is repeated with each color, producing the four-color proof. Pre-coated pigment carrier sheets are available only in certain colors, unless specially ordered. As manifest, such color proofing procedure, although providing a single sheet color proof, is of limited applicability and is inherently rather slow in cycle time in that each pigment layer of the four-color proof must be separately and successively laminated, exposed and developed.
3M Company also markets a color proofing system under its trademark "Matchprint II" which allegedly retains the advantages of "Transfer-Key" proofs, but incorporates a single level of optical gain in order to try and accurately simulate the press gain normally encountered in today's high speed web publication printing industry.
Another commercially available pre-press color proofing system is marketed by the DuPont Company, under the trademark "Cromalin". The film used therein has an originally sticky surface which when selectively exposed to light becomes nonsticky in the exposed area. Pigment toners are then rubbed onto the surface and stay in the sticky areas. This provides little flexibility in varying dot size and density. In the "Cromalin" color proof system, dry pigment toners are factory "calibrated" to printing ink colors and each primary and black color reproduction is on a separate sheet of photopolymer film, the films being laminated, exposed and toned one layer after the other. The cycle time for this system is said to be "within an hour", rather than the hours or days required for press proofing.
One serious limitation of all of these commercially available systems is that they have a fixed dot gain and therefore can only simulate combinations of printing presses and media which have similar levels of dot gain. Another limitation is that the proofs provided by these systems are not always as consistent as is desired by some in the graphic arts industry.
It has been recognized that dot gain observed in press sheets can be controlled to some extent by varying the amount of light passing through a separation when exposing a press plate. Increasing light intensity and/or exposure time increases dot gain where the final positive press sheet is produced from a printing plate formed from a negative separation. Increasing light intensity and/or exposure time decreases dot size where the final positive press sheet is produced from a positive separation. The article entitled "Research Completed on Modifying Dot Gain", Offset Printers, No. 162; July, 1982, page 25, states: "The dot gain occuring between film and print can be significantly reduced by increased exposure during printing plate making. The resulting print, however, shows disproportionate sharpening in the highlight and middle tone when compared to the shadow tones. It is not, therefore, recommended as an ideal way to compensate for excessive gain."
In some instances a proof press plate is formed from initial separations and a proof press sheet is printed. If the press proof is not an acceptable reproduction of the artwork, the separations are modified. A new proof press plate is formed from the modified separations and another press sheet is printed. This process is repeated until the resulting press sheet is an acceptable reproduction of the artwork. The proof press printing plate is not always an acceptable reproduction of a production printing plate and the proof press does not always perform in an identical manner to the production printing press. Accordingly, even though the proof press sheet may have an acceptable relationship to the original artwork, the production press sheet may not. In addition, the use of proof press plates and proof press printing therefrom is a very expensive and time consuming process.
It is therefore desirable to be able to consistently and relatively quickly and easily generate a color proof that has an acceptable relationship to the artwork and to a press sheet.